Little Six-arm Brittle Star

Overview

The Little Six-arm Brittle Star (Ophiactis savignyi) is widely considered the most common and broadly distributed brittle star in the world. Thriving in tropical and subtropical waters across the globe, this small echinoderm is frequently found in high densities within complex marine environments.

Distribution & habitat

This species is found throughout the tropical and subtropical regions of the Atlantic, Pacific, and Indian Oceans. It is highly adaptable and commonly inhabits marine fouling communities, including sponges, coral rubble, algae, seagrass beds, and mangrove areas. It is also known to colonize artificial structures such as ship hulls, docks, and buoys.

Appearance

Typically measuring only 2.5 to 10 mm in disk diameter, this is a very small brittle star. Unlike many other species that have five arms, O. savignyi usually possesses six. Its arms are long, slender, and banded with greenish-brown tones, often featuring small, thorny spines. The central disk is covered in overlapping scales and large, dark radial shields that touch each other near the arm bases.

Biology & behavior

O. savignyi is notable for its ability to reproduce both sexually and asexually. Asexual reproduction occurs through fission, where the central disk divides, allowing each half to regenerate missing parts. This process enables the species to rapidly colonize new areas. They are often found tucked inside sponges, where their arms may extend outward, giving the host sponge a "furry" or "hairy" appearance.

Feeding

This species acts as a deposit feeder, consuming detritus, foraminifera, and other small benthic invertebrates found within its microhabitat.